This invention relates to a flow control valve used in a fluid pressure system, e.g. in a brake system for automobiles, which a high-pressure, low-viscosity fluid has to be controlled precisely over a wide range of flow rates.
FIG. 5 shows a flow control valve proposed in Japanese Unexamined Patent Publication 3-223578 by the present applicant. It comprises a housing 1 having a first port 2 and a second port 3, a spool 4 slidably mounted in the housing, a first fluid chamber 5 and a second fluid chamber 6 in which fluid pressure can be generated on both ends of the spool from opposite directions, a fluid passage 8 having an orifice 7 through which the first and second fluid chambers 5 and 6 communicate with each other, the second fluid chamber 6 further communicating with the second port 3, and a valve portion 9 controlling the fluid flow between the first port 2 and the first fluid chamber 5 as the spool 4 moves in an axial direction.
With the automatic opening adjustment effect of the valve portion 9 in this flow control valve, a thrust to the spool produced by the difference in fluid pressure in the first and second fluid chambers 5 and 6 balances with the biasing force of biasing means 10. The flow determined by the difference in pressure and the sectional area of the orifice is introduced from the first port 2 through the orifice 7 to the second port 3.
In the conventional flow control valve, when the valve portion is closed, since the path from the first port to the first fluid chamber is closed by the annular seal, the flow rate can be controlled precisely even in low-flow area. However, some amount of leakage is inevitable through the gap between the spool and the housing from the first port to the second fluid chamber. Although the leakage can be reduced by narrowing the gap, it is impossible to reduce it to zero. Therefore, the advantage of precise control by use of the annular seal is not sufficiently utilized. The application of the flow control valve is thus limited.